1. Field of the Invention
Digital demodulation receivers are typically frequency-shift keying (FSK) receivers which receive periodic input signals of a first frequency and of a second frequency, providing a binary 1 output in response to the first frequency input and a binary 0 in response to the second frequency input. More particularly, the digital filter of this invention is utilized in an FSK receiver which handles the incoming signals in digital fashion.
2. Prior Art
In the past, the conversion of an input signal of two different frequencies into output binary 1 or 0 (often referred to as "mark" and "space," respectively) is accomplished in largely analog fashion.
For example, in a well known circuit, a pair of selective bandpass filters are each tuned to one of the two input frequencies. A comparison circuit compares the output energy of the filters and if the energy in the 1 bandpass filter is greater than the energy in the 0 bandpass filter, the output is given as a 1. The converse is true for a 0 indication. This circuitry requires high Q's for each filter and that its parameters remain extremely stable under shock, temperature and vibration conditions. Components required are high precision and therefore expensive. Factory adjustments are required to align the filters and periodic maintenance is performed to keep the center frequency of each properly positioned.
Another approach is the zero crossing detector which relates numbers of zero crossings per unit of time to either a 1 or a 0 output. This circuit is not limited to zero volts of course, but is applicable to any reference voltage. A linear filter removes the high frequency components from the detected signal and passes only the 1- 0 data. The output of the filter, however, does not have sharp data transitions and a limiting amplifier or comparator must be added.
A more recent and popular circuit is the phase locked loop which automatically locks onto the received signal and indicates 1's and 0's by levels in the control voltage. This circuit requires precision components and the accuracy of the center frequencies designed into the system is in the order of 1 to 5 percent. Also, the output does not have sharp data transitions and a limiting amplifier or comparator is required.
A digital frequency-shift keying receiver as described and claimed in copending patent application, Ser. No. 374,594, filed June 28, 1973, .Iadd.now U.S. Pat. No. 3,879,665 .Iaddend.assigned to the assignee of this invention, represents a different approach. The digital filter described herein may advantageously be utilized in a digital FSK receiver for reducing bias distortion which is brought about by counting an up-down counter in one direction at a first frequency and in the other direction at a second frequency. In the instant invention, an up-down counter is continuously conditioned by a signal from a storage unit representing a binary 1 or a binary 0 input to the digital FSK receiver while a clock source, independent of the input frequency is used for causing the up-down counter to count in one direction to a threshold count if a 1 input is present, and to count in the other direction to a threshold count if a 0 input signal is present.